Add Notification.maxActions so websites know how many buttons we support

Source/modules/notifications/Notification.h

Index: Source/modules/notifications/Notification.h
diff --git a/Source/modules/notifications/Notification.h b/Source/modules/notifications/Notification.h
index e922eb9227fbdd6c7178da3884fdc23287de9b70..86b7fcb0d3cd31e4babf734ffceb4e913efc3fc7 100644
--- a/Source/modules/notifications/Notification.h
+++ b/Source/modules/notifications/Notification.h
@@ -103,6 +103,8 @@ public:
     static WebNotificationPermission checkPermission(ExecutionContext*);
     static void requestPermission(ExecutionContext*, NotificationPermissionCallback* = nullptr);
 
+    static unsigned maxActions();
+
     // EventTarget interface.
     ExecutionContext* executionContext() const final { return ActiveDOMObject::executionContext(); }
     const AtomicString& interfaceName() const override;